The goal of this work was to determine the concentration of selenium in biological materials samples (urine, hair, and serum) of 500 residents from 5 towns and 3 villages in eastern Croatia. Selenium concentrations were also determined in some of the collected samples of water, plants, and soil. The analyzed results for the median span: serum from 80,75 μg \(L^{-1} \) till 113,92 μg \(L^{-1} \), urine from 12,44 μg \(L^{-1} \) till 36,31 μg \(L^{-1} \) , hair from 0,20 μg \(g^{-1} \) till 0,60 μg \(g^{-1} \). Mean: water 1,95 μg \(L^{-1} \), soil from 0,02 till 0,36 μg \(g^{-1} \), vegetables (Cabbage) from 22,61 till 201 μg \(kg^{-1} \), Dandelion from 18,30 till 133,4 μg \(kg^{-1} \) . The research was carried out on a large number of samples, therefore the results of this paper contribute to the significant expansion of the reference database for Croatia.